Carbon fibers are generally produced by a well-known process where raw fibers (precursor fibers) such as polyacrylonitrile (PAN) are oxidized and carbonized to give carbon fibers. The carbon fibers thus obtained have excellent properties such as high strength and high elastic modulus.
Composite materials (for example, carbon fiber reinforced plastic (CFRP)) produced utilizing carbon fibers have been used for increasing applications. For example, in the fields of sports/leisure, aerospace and automobiles, (1) improved performance (improvement in strength and elasticity) and (2) weight reduction (weight reduction of fibers and reduction of a fiber content) have been required in a composite material. For meeting these requirements, there has been needed carbon fibers which can give a composite material exhibiting improved physical properties by combining carbon fibers and a resin (matrix material).
For providing a high-performance composite material, physical properties of the matrix material are important. Improving the surface properties, strength and an elastic modulus of carbon fibers is also important. Generally, it is important to combine a matrix material and carbon fibers having a carbon fiber surface exhibiting high adhesiveness to the matrix material, and to adequately uniformly disperse the carbon fibers in the matrix material. Thus, a higher-performance composite material can be provided.
There have been investigations for surface crease, surface properties, strength and an elastic modulus of carbon fibers (for example, see Patent References Nos. 1 to 4).
In producing carbon fibers, a spinneret having more spinning holes is more suitable for large-scale production. However, a precursor fiber strand produced by spinning from a spinneret having 20,000 or more spinning holes has higher fiber-opening tendency, if nothing is done. Therefore, when a carbon fiber strand is produced using such a precursor fiber strand as a raw material, fiber opening excessively proceeds during the oxidation and the carbonization steps described later to provide a carbon fiber strand exhibiting inconsistent physical properties.
When a large amount of a sizing agent is added for controlling an extent of fiber opening, particularly in the carbonization step, there generate a large amount of impurities derived from the sizing agent, leading to a highly uneven carbon fiber strand, so that a carbon fiber strand with high strength and a high elastic modulus cannot be provided.
To avoid the above problems, there is proposed a process for producing a precursor strand consisting of 20,000 or more single fibers by bundling a plurality of precursor strands spun using a spinneret having a relatively smaller number of spinning holes.
An example is thought of production of a carbon fiber strand as a bundle of 24,000 single fibers. Generally, a precursor strand consisting of 3,000 to 12,000 single fibers can be provided using one spinneret. Two to eight of the precursor strands can be collected into a precursor strand consisting of 24,000 single fibers, which can be then oxidized and carbonized to give a carbon fiber strand consisting of 24,000 single fibers. Alternatively, each of the precursor strands can be directly oxidized and then, the individual strands can be collected during the subsequent carbonization to give a carbon fiber strand consisting of 24,000 single fibers. Alternatively, each of the precursor strands can be directly oxidized and then carbonized before collecting the individual strands to give a carbon fiber strand consisting of 24,000 single fibers.
However, when a composite material is produced using carbon fiber strands prepared as described above, fiber opening of the collected carbon fiber strands for resin impregnation substantially causes separation of these into the original strands, which is so-called strand splitting.
Since each carbon fiber constituting a collected strand is not prepared from a single spinneret, its properties such as strength tends to significantly vary.
As described above, in a carbon fiber strand consisting of 20,000 or more single fibers prepared by collecting a plurality of strands, strand splitting tends to occur during fiber opening and physical properties of each carbon fiber constituting a strand are inconsistent. Furthermore, since physical properties of each carbon fiber constituting a strand are inconsistent, a strand tensile strength and a strand tensile modulus of the carbon fiber are generally low.
Generally, for producing a composite material, a carbon fiber strand is adequately fiber-opened and then, uniformly impregnated with a matrix resin. When strand splitting occurs during fiber opening of the carbon fiber strand, impregnation with the resin becomes uneven, leading to deterioration in physical properties of the composite material obtained. Therefore, the feature required for a carbon fiber strand suitable for manufacturing a composite material is adequate fiber opening without causing strand splitting.    Patent Reference No. 1: Japanese published unexamined application No. 1998-25627 (Claims).    Patent Reference No. 2: Japanese published unexamined application No. 2006-183173 (Claims).    Patent Reference No. 3: Japanese published unexamined application No. 2005-133274 (Claims).    Patent Reference No. 4: Japanese published unexamined application No. 2002-327339 (Claims).